1. Field of the Invention
The present invention generally relates to generating satellite orbit information for earth orbiting satellites. More specifically, the invention relates to a method and apparatus for compressing long term orbit information (also known as “extended ephemeris” information) prior to distributing the information through a network or communications link.
2. Description of the Related Art
A positioning receiver for a Global Navigation Satellite System (GNSS), such as the Global Positioning System (GPS), uses measurements from several satellites to compute a position. The process of acquiring the GNSS radio signal is enhanced in speed and sensitivity if the GNSS receiver has prior access to a model of the satellite orbit and clock. This model is broadcast by the GNSS satellites and is known as an ephemeris or ephemeris information. Each satellite broadcasts its own ephemeris once every 30 seconds. Once the GNSS radio signal has been acquired, the process of computing position requires the use of the ephemeris information.
The broadcast ephemeris information is encoded in a 900 bit message within the GNSS satellite signal. It is transmitted at a rate of 50 bits per second, taking 18 seconds in all for a complete ephemeris transmission. The broadcast ephemeris information is typically valid for 2 to 4 hours into the future (from the time of broadcast). Before the end of the period of validity the GNSS receiver must obtain a fresh broadcast ephemeris to continue operating correctly and produce an accurate position. It is always slow (no faster than 18 seconds), frequently difficult, and sometimes impossible (in environments with very low signal strengths), for a GNSS receiver to download an ephemeris from a satellite. For these reasons it has long been known that it is advantageous to send the ephemeris to a GNSS receiver by some other means in lieu of awaiting the transmission from the satellite. Previously available systems use a technique that collects ephemeris information at a GNSS reference station, and transmits the ephemeris to the remote GNSS receiver via a wireless transmission. This technique of providing the ephemeris, or equivalent data, to a GNSS receiver has become known as “Assisted-GNSS”. Since the source of ephemeris in Assisted-GNSS is the satellite signal, the ephemeris information remains valid for only a few hours. As such, the remote GNSS receiver must periodically connect to a source of ephemeris information whether that information is received directly from the satellite or from a wireless transmission. Without such a periodic update, the remote GNSS receiver will not accurately determine position.
Until recently, the deficiency of the current art was that there was no source of satellite trajectory and clock information that is valid for longer than a few hours into the future, and it can be expensive to send the ephemeris information repeatedly to the many remote devices that may need it. Moreover, mobile devices may be out of contact from the source of the Assisted-GNSS information when their current ephemeris becomes invalid. Consequently, long term orbit models (sometimes referred to as extended ephemeris) are used to enable an assisted-GNSS receiver to operate for a long period of time before new ephemeris is required. However, long term orbit models that extend over a long period of time can contain many bits that form large files. These large files utilize extensive bandwidth when being transmitted through a network to a GNSS receiver.
Therefore, there is a need in the art for a method and apparatus for providing satellite trajectory and clock information that is valid for an extended period into the future, e.g., many days into the future, and sending that information to a GNSS receiver in a compressed form.